1. Field of the Disclosure
The present disclosure is generally directed to systems and methods for treating oilfield wastewater, and in particular to mobile systems that may be used to treat wastewater that is generated from a variety of sources during drilling and completion operations.
2. Description of the Related Art
Large amounts of water are often necessary to facilitate the drilling and completion activities performed on modern oil and gas wells. In many cases, an even greater amount of wastewater can be generated that during these operations must be handled in some fashion, such as by treatment, reuse, disposal, and the like. The wastewater generated during drilling and/or completion operations can come from a variety of different sources, each of which may have different components and/or characteristics that may have an influence on how the wastewater is handled.
For example, one major source of oilfield wastewater is so-called “produced water,” which generally is naturally-occurring water that is trapped in underground formations where oil and gas are found, and that comes to the surface during oil and gas exploration and production. Because the water has been in contact with the hydrocarbon-bearing formation for an extended period of time—millions of years, in most cases—it contains some of the chemical characteristics of the formation as well as that of the hydrocarbon itself. When extracted, the oil or gas is brought to the surface along with this produced water as a combined fluid. As such, the composition of the produced fluid will typically include a mixture of liquid and/or gaseous hydrocarbons, produced water, dissolved or suspended solids, produced solids such as sand or silt, and other fluids or chemicals that may have been injected into the formation during exploration and production activities. Therefore, produced water is not simply a single recognizable commodity, as its physical and chemical properties can vary considerably depending on the geographic location of the field, the geological host formation, and the type of hydrocarbon product being produced. Furthermore, the properties of produced water, as well as the volume of produced water, can vary throughout the life of given well or reservoir.
The U.S. Department of Energy (DOE) has characterized produced water as “by far the largest volume byproduct or waste stream associated with oil and gas exploration and production.” As recently as 2009, studies have shown that approximately 21 billion barrels of produced water are generated each year in the United States from nearly a million wells—which represents about 57 million barrels per day, or 2.4 billion gallons per day—and that more than 50 billion barrels of produced water are generated each year at thousands of wells in countries other than the United States.
Early in the life of a well, oil production is typically high and water production is low. However, over the life of a well, oil production will generally decrease, whereas water production will increase. The DOE estimates that the average ratio of produced water-to-oil on a worldwide basis is in the range of 2:1 to 3:1, however the estimated U.S. average falls somewhere between 5:1 and 8:1. This is due to the fact that many U.S. fields are more mature and therefore past their peak production, although the actual average ratio in the U.S. could be even higher, because produced water is not always measured directly. Furthermore, a produced water-to-oil ratio in excess of 50:1 can be found in many older wells in the U.S. and elsewhere.
Another major source of wastewater that is generated during oil and/or gas well drilling operations is “flowback” or “frac water” from hydraulic fracturing operations. Hydraulic fracturing is a common enhancement method that is often used for stimulating the production of oil and/or gas, particularly in hydrocarbon-bearing shale formations. The fracturing fluid, sometimes referred to as “fracing fluid,” is typically a mixture of water and proppant particulates, which may include sand and/or synthetic material, as well as a variety of chemicals that are often added so as to aid in proppant transport, friction reduction, wettability, pH control, bacterial control, and the like. The fracing process involves injecting a fracing fluid down a well bore at a sufficiently high hydraulic pressure so that the fracing fluid penetrates the producing formation and creates underground cracks or fractures in the formation, or extends any existing fissures. The proppant particulates then acts to “prop” the cracks or fissures open, thus allowing at least some of the fracing fluid to “flow back” into the wellbore and out of the well, along with some amount of the naturally-occurring water that may be present in the formation. In many instances, the fracing process is repeated a multiple number of times on a given well, in which case the well head is closed between stages so to maintain the hydraulic pressure of the fracing fluid for an extended period of time.
The flowback or frac water flows back to the surface during and after the completion of the hydraulic fracturing operation. The composition of flowback is generally characteristic of the original fracing fluid that is used to fracture the formation, however the physical and chemical properties of flowback can vary considerably depending on the geographic location of the well, the specific makeup of the geological formation, and any other chemicals that might be introduced into the well during the drilling and fracturing operations. Most flowback will occur within the first seven to ten days of the fracing operation, while the rest can occur over a period of three to four weeks or more.
Varying amounts of water are required in a typical hydraulic fracturing operation, however it is not uncommon to use anywhere from one to four million gallons of water to fracture the formations of a single oil or gas well. Furthermore, some wells may require even greater quantities of water depending on the length of the fracture and the depth of the frac zone. Generally, the water is trucked or piped to the well site from other locations, typically in very large quantities, and may come from a variety of different sources, including untreated water from rivers, lakes, streams, ponds, or water wells, or it may be purchased from a municipal water utility. Accordingly, it should be appreciated that there is a substantial cost associated with obtaining, shipping, and storing the water that is used for performing a typical fracing operation.
While produced water and flowback often represent a significant percentage of the total amount of wastewater that is generated during oil and gas well operations, other sources and operations can also contribute to the generation of wastewater. For example, some amounts of wastewater are typically generated when plugs that are used to separate the various production zones of a well are drilled out, sometimes referred to as “coiled tubing water” or “coiled tubing drill-out waste.” Various other rig operations may also contribute to the generation of wastewater, such as drilling fluid treatment waste, storage impurity sediments, spent lubricants, and/or well servicing fluids and the like, which are sometimes generically and collectively referred to as “pit water.” In general, most if not all of the wastewater that is generated during these various operations—e.g., produced water, flowback, coiled tubing water, pit water, etc.—is pumped to and held in large holding ponds or tanks that are created adjacent to the well site. Handling of the wastewater thereafter generally depends on its subsequent disposition, such as by disposal and/or reuse.
For example, in some cases, the most economical manner of handling the wastewater may simply be to dispose of the wastewater, either by reinjecting it into commercial underground reinjection sites or by discharging it into waterways. In other cases, it may be desirable to reuse at least some of the wastewater for operations at the well site, such as for further fracing operations and the like. However, irrespective of the ultimate disposition method—i.e., by disposal and/or reuse—some amount of treatment of the wastewater is generally required.
Treating wastewater from oil and gas operations that is primarily composed of produced water and flowback often entails removing a variety of different chemical constituents and dissolved solids from the wastewater. Furthermore, the degree to which these chemical constituents and solids must be removed is generally dependent on the treated wastewater disposition method. Accordingly, while removing only minimal amounts of the various chemical constituents and total dissolved solids from the wastewater may render it marginally suitable for disposal by reinjection it into underground sites, such minimal treatment may be insufficient to allow the treated wastewater to be reused in hydraulic fracturing operations, which typically requires a more extensive wastewater treatment. Similarly, the level of contaminant removal that is necessary to permit the treated wastewater to be discharged into waterways generally well exceeds the commensurate levels that that may be allowed for both reinjection disposal and fracing reuse. Accordingly, a variety of different water treatment options are often necessary in order to meet all of these competing objectives.
Furthermore, the method of disposition and the degree of treatment often dictates whether or not the wastewater can be economically treated on site, due to limited treatment capabilities of most commonly available mobile wastewater treatment systems. For example, when only minimal treatment of the wastewater is required, such as when the wastewater is ultimately disposed of by way of subterranean reinjection, it may be feasible to perform such minimal treatment at the well site prior to trucking the wastewater to the reinjection sites. However, other than systems that utilize very large, high residence time settling tanks—which typically have a very large well site footprint—most commercially available mobile treatment systems are generally incapable of treating wastewater from oil and gas operations to a sufficient degree that would allow the water to be reused for fracing operations, much less to a level that would permit the treated water to be discharged in waterways and the like. In such cases, the wastewater must first be trucked from the well site to water treatment facilities, after which it must be trucked back to the well site for reuse in fracing operations, or trucked to separate disposal sites. In either instance, the costs associated with trucking the wastewater to offsite facilities for treatment can often have a significant impact on the economic viability of the well. Furthermore, the large quantities of water required for hydraulic fracing, together with the additional chemicals and/or other contaminants that are often present in fracing water flowback, only tend to exacerbate the wastewater treatment issues that have long been associated with produced water and other wastes from oil and gas operations. Accordingly, operators are continuously seeking ways to reduce the costs and environmental impact associated with handling and treating the large amounts wastewater that are generated during oil and gas exploration, drilling, and completion activities.
The present disclosure is therefore directed to methods and systems for treating oil and gas wastewater that may be used to address one or more of the various issues outlined above.